The metabolic regulation and mechanism of catalysis of the important biosynthetic enzyme phosphoribosylpyrophosphate (PRPP) synthetase from Salmonella typhimurium will be studied. The mechanism of allosteric regulation will be clarified by ligand binding and other physical and kinetic studies. The roles of divalent cations, steric arrangement of substrates at the active site, and the stereochemical course of catalysis are being investigated by kinetic and magnetic resonance techniques, using substrate analogues. Chemical studies are projected to clarify an unusual quaternary structure and to identify active site residues. Techniques have been devised which enable a search for a mutant with a thermolabile PRPP synthetase. These studies will contribute to understanding the mechanism of catalysis by nucleotide-dependent transferases in general and will clarify the regulation of PRPP synthesis and metabolism. This latter subject is of particular interest because of the importance of PRPP in human metabolic diseases of purine and pyrimidine metabolism.